As a radiation detector using gas amplification, a pixel-type radiation detector has been used conventionally. This radiation detector employs a constitution such that a strip-shaped cathode electrode is formed on a front surface of a double-sided printed-circuit board, for example, an anode strip is formed on a back surface thereof, opening portions are formed in the strip-shaped cathode electrode at regular intervals, and a column-shaped anode electrode connected to the anode strip on the back, namely a pixel electrode is formed at the center of each of the opening portions.
Incidentally, the above-described radiation detector is disposed in a mixed gas of argon and methane, for example. Further, a voltage of +600 V, for example, has been applied to the above-described pixel electrodes.
In the above-described radiation detector, when predetermined radiation enters the aforementioned detector, the aforementioned gas is ionized to generate electrons and these electrons cause electron avalanche amplification by a large voltage applied between the above-described strip-shaped cathode electrode and the above-described pixel electrode and a strong electric field generated due to a shape (shape anisotropy) as a point electrode of the above-described pixel electrode. On the other hand, positive ions generated by the aforementioned electron avalanche amplification drill toward the aforementioned strip-shaped cathode electrode around the ions.
As a result, the strip-shaped cathode electrode and the pixel electrode to be a target are each charged with the positive ion and the electron. Thus, detection of the positions of the strip-shaped cathode electrode and the pixel electrode in which electric charges are generated in this manner makes it possible to specify an incident position of radiation in the detector, resulting in that radiation detection is enabled (Patent Document 1).
In the above-described radiation detector, when the voltage to be applied to the pixel electrode is increased, the intensity of the electric field to be generated also increases and the above-described electron avalanche amplification becomes significant, and thus an amount of the electric charges to be generated in the strip-shaped cathode electrode and the pixel electrode increases and sensitivity (a gas amplification factor) of radiation improves. On the other hand, when the voltage to be applied to the pixel electrode is increased, by abnormal discharge caused by the shape of a via hole conductor bonded to the pixel electrode, the pixel electrode and the via hole conductor break in some cases. Further, when the voltage to be applied to the pixel electrode is reduced, the above-described abnormal discharge decreases, but the degree of the above-described electron avalanche amplification also decreases and detection sensitivity of radiation decreases.
From such a viewpoint, narrowing the via hole conductor bonded to the pixel electrode and improving the intensity of the electric field to be generated have been attempted in place of increasing the voltage to be applied to the pixel electrode. However, the above-described via hole conductor is formed by performing via filling plating inside a through hole formed in the printed-circuit board, and thus narrowing the aforementioned through hole is also required in order to narrow the via hole conductor. On the other hand, narrowing the through hole makes it impossible to uniformly perform the via filling plating inside the aforementioned through hole and to form an uniform via hole conductor, resulting in that problems such as abnormal discharge, dielectric breakdown, and sensitivity variation (risk of an increase in low-sensitivity pixel) occur in the aforementioned pixel electrode. Therefore, narrowing the aforementioned via hole conductor relays on its manufacturing method to be limited naturally (Patent Document 2).
In place of increasing the voltage to be applied to the pixel electrode, attaching a GEM (Gas Electron Multiplier) to the radiation detector has also been attempted similarly, but there has also been a problem of failing to perform radiation detection stably because of occurrence of variations in amplification caused by complicated installation of the GEM.
As a result, there has been a problem of failing to sufficiently improve the sensitivity (gas amplification factor) of the above-described pixel-type radiation detector at present.